For standard residential branch circuits at 30°C ambient, use the 60°C or 75°C column of the NEC 310.16 amperage chart. 14 AWG handles 15A, 12 AWG handles 20A, 10 AWG handles 30A, and 8 AWG handles 40A. Always size the breaker to the lowest temperature rating of any connected device, which is typically 60°C for circuits 100A and below. If you are pulling THHN in conduit, you use the 90°C column only for derating calculations, but your final breaker size must still respect the termination limits.

How to Read This Copper Wire Amperage Chart

The most common mistake DIYers and junior electricians make is looking at the 90°C column because they bought THHN wire and assuming they can use that higher ampacity for breaker sizing. You cannot. The National Electrical Code (NEC) dictates that your wire ampacity is limited by the weakest link in the circuit, which is almost always the termination lug on the breaker or receptacle.

Under NEC 110.14(C), you must select your column based on the equipment's temperature rating:

  • 60°C Column: Use this for circuits rated 100A or less, or for 14 AWG through 1 AWG wire, unless the equipment is explicitly marked otherwise. This is the default column for NM-B (Romex) cable, as its ampacity is permanently limited to 60°C regardless of the conductor's actual insulation rating.
  • 75°C Column: Use this for circuits over 100A, or for 1/0 AWG and larger. You can also use it for smaller wires if the breaker, lug, and receptacle are all explicitly stamped '75°C'.
  • 90°C Column: Use this only as the starting base value for derating calculations (like ambient temperature adjustments or bundling). The final derated ampacity must then be compared to the 60°C or 75°C column, and you must use the lower of the two values.
Bench Tip: If you are wiring a 20A receptacle using 12 AWG THHN in a conduit, the 90°C column says 30A. However, the receptacle terminals are rated 60°C. Therefore, your maximum allowable ampacity for breaker sizing remains 20A (from the 60°C column). The 90°C rating just gives you 'headroom' for derating if you have multiple wires in that conduit.

The Master Amperage Chart (NEC Table 310.16)

The following table is derived directly from NEC 2023 Table 310.16 (formerly 310.15(B)(16)) for copper conductors. It assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. Bookmark this section for quick jobsite lookups.

Copper Conductor Ampacities (Source: NEC Table 310.16, 30°C Ambient)
AWG / kcmil 60°C (140°F)
NM-B / Default Terminations		 75°C (167°F)
THWN / High-Temp Lugs		 90°C (194°F)
THHN / Derating Base
14 AWG (Quick Jump: 15A)15A20A25A
12 AWG (Quick Jump: 20A)20A25A30A
10 AWG (Quick Jump: 30A)30A35A40A
8 AWG (Quick Jump: 40A)40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A115A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

Derating: When the Base Amperage Drops

The amperage chart above assumes ideal conditions: 30°C ambient air and a maximum of three current-carrying conductors bundled together. When you deviate from these conditions, the wire cannot dissipate heat as efficiently, and you must apply derating multipliers to the 90°C column base value. For a comprehensive breakdown of bundling rules, refer to the ECMWeb National Electrical Code resource center.

1. Ambient Temperature Derating (NEC Table 310.15(B)(1))
If your conduit runs through an attic that reaches 46°C (115°F), you must multiply the 90°C base ampacity by 0.82.
Example: 10 AWG THHN has a 90°C base of 40A. 40A × 0.82 = 32.8A. Since 32.8A is greater than the 60°C column limit of 30A, you can still safely use this wire on a 30A breaker.

2. Bundling Derating (NEC Table 310.15(C)(1))
When you pull more than three current-carrying conductors in a single raceway, the heat compounds. Note that a neutral conductor carrying only unbalanced load from a standard single-phase circuit does not count, but a neutral on a 3-phase wye circuit with high harmonics does.

  • 4 to 6 conductors: Multiply base ampacity by 80%.
  • 7 to 9 conductors: Multiply base ampacity by 70%.
  • 10 to 20 conductors: Multiply base ampacity by 50%.
War Story: I once saw an apprentice pull four 12 AWG THHN wires (two 20A circuits) in a single EMT conduit and protect them with 20A breakers. The 90°C base is 30A. Derated at 80% for 4 conductors, the ampacity drops to 24A. While 24A is technically above the 20A breaker, the 60°C termination limit for 12 AWG is strictly 20A. It passed inspection, but if that conduit had been in a hot attic, the compounding ambient derating would have forced an upsize to 10 AWG.

Decision Path: Pick Your Exact Wire Size

Use this decision-tree-table to terminate your sizing process with a concrete pick. Do not guess; follow the logic path based on your continuous load and installation method.

If Your Max Load Is... And Your Installation Is... Then Buy This Wire & Breaker
15 Amps (Standard Lighting) NM-B (Romex) in walls 14 AWG NM-B on a 15A breaker.
20 Amps (Kitchen/Bath Receptacles) NM-B or THHN in conduit 12 AWG on a 20A breaker. (Use 12 AWG even if load is only 16A to prevent voltage drop on long runs).
30 Amps (Dryer / RV Outlet) THHN in conduit, < 4 conductors 10 AWG THHN on a 30A breaker.
40 Amps (Range / EVSE / Subpanel) THHN in conduit, < 4 conductors 8 AWG THHN on a 40A breaker. (If using NM-B, you must upsize to 8 AWG, but 8 AWG NM-B is rated 40A, so it works).
50 Amps (Welder / Hot Tub) THHN in conduit, < 4 conductors 6 AWG THHN on a 50A breaker. (Note: 6 AWG NM-B is only rated 55A at 60°C, which is fine for a 50A breaker, but THHN is easier to pull).
60 Amps (Subpanel Feeder) THHN in conduit 6 AWG THHN (75°C col = 65A) on a 60A breaker.
100 Amps (Main Subpanel) THHN in conduit or SER Cable 3 AWG THHN or 2 AWG Aluminum SER on a 100A breaker.

What This Amperage Chart Cannot Tell You

While the NEC 310.16 amperage chart is the gold standard for thermal limits and breaker sizing, it is blind to three critical real-world factors. If you ignore these, your circuit might be 'code compliant' but functionally useless.

  1. Voltage Drop: The chart assumes you are only protecting the wire from melting. It does not account for resistance over distance. If you run 12 AWG wire 150 feet to a 15A load, the wire won't overheat, but the voltage at the receptacle will drop below 114V, causing motors to overheat and electronics to brownout. For runs over 75 feet on a 120V circuit, always upsize one AWG to maintain the recommended 3% maximum voltage drop.
  2. Short-Circuit Withstand: Ampacity measures continuous thermal loading. It does not tell you if the wire can survive the magnetic and thermal forces of a 10,000A short circuit before the breaker trips. For standard residential panels with 10kAIC ratings, standard AWG sizes are fine, but in industrial settings with high fault currents, you must verify short-circuit withstand ratings.
  3. Physical Fit and Bend Radius: A 4/0 AWG wire might be rated for 195A, but if you are trying to land it in a small 100A disconnect switch designed for a maximum of 1/0 AWG, it physically will not fit. Always check the manufacturer's lug sizing data sheet before purchasing large-gauge feeders.

Final Default Recommendation: When calculating loads that fall exactly on the boundary of a wire's ampacity, or when you are unsure about the temperature rating of the equipment you are terminating into, always default to the 60°C column and upsize to the next physical AWG thickness. The marginal cost of 100 extra feet of 10 AWG over 12 AWG is roughly $15, but it guarantees you will never fail an inspection or overheat a termination lug.